In-Situ Measurement of Vertical Distribution of Radiocaesium Concentration in Fukushima Soil.

On March 11, 2011, great earthquake hit in northeast Japan that caused Fukushima Daiichi Nuclear Power Plant accident. Although radiocaesium (¹³⁴Cs, ¹³⁷Cs) was fixed strongly to the soil from surface to 3〜5 cm in depth, agricultural fields have been disturbed by wild boar and weed after three years  from the accident. For the effective decontamination of the agricultural fields, it is needed to measure the vertical distribution of radiocaesium concentration in the soil in a short time. In order to measure the vertical distribution of radiocaesium concentration in Fukushima soil, we have developed an in-situ device “DOJYO-kun” which is 8 cm in diameter and 10 cm length with four Geiger-Mullar tubes (LND 712). The device was installed for three minutes in a hole after soil core (5 cm diameter, 16 cm long) was taken an undisturbed paddy field in Iitate Village, Fukushima before and after decontaminating by soil paddling method. Then, the applicability of the device was evaluated by comparing cpm (count per minute) of the device and analyzed radiocaesium concentration of the soil core by NaI-scintillator in each depth. As a result, the device could detect the decrease of radiocaesium concentration of soil after decontaminating in the agricultural fields. However, some significant differences were found in the deep layer from the surface between the device and NaI-scintillator. These differences are probably attributed to insufficient evaluations of radiation’s scattering and attenuation, disturbance of the top soil when we set the device. However, the device is promising to monitor trends and decontamination effects in a short time in the agricultural field in Fukushima. More precise calibrations are needed to measure radiocaesium concentration at each depth accurately.